1. Field of the Invention
This invention relates to torque compensation for pitch momentum biased satellites in equatorial or low inclination orbits and more particularly for the compensation of constant or very slowly varying roll disturbance torques by magnetic torquing in an open loop system.
2. Description of the Prior Art
A stabilized orbiting satellite requires a means for changing its attitude when it has deviated from its desired orientation or position relative to its orbit.
Magnetic torquing of dual-spin stabilized satellites is known. Such magnetic torquing systems use a magnetic field from torquers such as coils or electromagnets to interact with the magnetic field of the earth to develop a reaction torque. This reaction torque causes the reference axis of the satellite to be repositioned an amount proportional to the torquing time and flux magnitude as is well known in the art.
A closed loop system for controlling roll and yaw error in an orbiting pitch momentum biased satellite is described in U.S. Pat. No. 3,834,653 issued Sept. 10, 1974, to RCA Corporation, based on the invention of Harold Perkel entitled "Closed Loop Roll and Yaw Control for Satellites." The system described therein achieves control by using the errors in roll as the control input parameter of the closed-loop system. A magnetic dipole oriented along the roll axis for a satellite in a synchronous low inclination orbit interacts with the primary magnetic field which is perpendicular to the orbit plane to produce a magnetic control torque about the yaw axis. Attitude sensors in combination with electronic logic energize the dipole with currents of appropriate polarity and magnitude to effect the necessary torque, to correct the satellite's attitude. Because of gyroscopic cross-coupling between the roll and yaw axes, the control torque causes precession about the roll axis which cancels out the roll errors. Thus, the magnetic torquer directly reduces the roll error and indirectly controls the yaw error.
As is well known in the art for such an earth oriented spacecraft, yaw errors are not sensible by earth sensors. Further, roll and yaw errors interchange sinusoidally on approximately a quarter orbit basis throughout the orbit for such a momentum biased spacecraft because of the inertial stiffness of the momentum vector. Thus, in the closed loop system described in the above patent, satellite motion resulting from constant or slowly varying roll disturbance torques will be compensated after a quarter orbit, when the resulting yaw motion has coupled into roll. This quarter orbit lag in achieving correction of roll disturbance torques may result in unacceptably large yaw errors.
For a spacecraft in either an equatorial orbit, such as a synchronous communication satellite, or a low inclination orbit, the principal cause of constant or very slowly varying roll disturbance torques is solar radiation pressure on the spacecraft's solar array or antenna farm. Roll disturbance torques can also result from other factors such as the gravity gradient or residual spacecraft magnetic dipoles.
Accordingly, a system for directly compensating for predicted slowly varying roll disturbance torques is needed to overcome the deficiencies of the presently known system.